TYRE PRESSURES

[nsparey]

Popped into the local ATS today to get the exhaust checked out and one of the front tyres seemed to wearing on the edges.

Exhaust was ok, but they checked the tyre pressures and found the front one only had 18 lbs in it, he checked the others and said they were all well down on what they should be, he reckoned 32lb in the front and 28lb in the back.

I then set off to our other branch which happens to be 26 miles of wonderful winding roads over the Preseli Mountains. I instantly noticed that the car was much sharper and I could put more power down coming out of corners and it felt much more stable at speed.

I notice this evening that the service sticker on the windscreen says 22lb front and 24lb rear, so now I am confused, what should they be? As it’s an SE, I have Toyo Proxies 225/16/50 front and 245/16/45 on the rears.


>>> Edited by nsparey on Monday 22 March 23:27

[julianhj]

I always ran 22psi front / 24psi rear, having taken advice from the PH collective.

[Guillotine]

i run toyo proxie 16" all round on my griff.

22 front 24 rear. warm. factory spec.

the car will have felt more pointy for sure, but any serious cornering and you'll have understeered off at a vast rate of knots.

as for the power, thats surprising. would of thought it'd want to slide much more easily. your contact patch on the road was probably 25 down on normal.

cheers

[nsparey]

I here what your saying and it makes perfect sense, I will set them back to factory settings today, I am a little worried about the front tyre wear though, is 4lbs down enough to knock the edges of a tyre in 400 miles? and I have to say that the fronts always looked a little "saggy", but to my eyes the rears look over tyred to, but I guess they work.

[jam1et]

Hard cornering can scrub off the inside edges, but not usually within 400 miles standard road use. I'd have the geometry checked.
I also run 22, 24 (checked when warm). Its worth getting your own tyre gauge and foot pump too. Just to experiment, I filled my tyres at a garage the other day then used my own gauge (accurate to 0.5psi) and there was upto 2psi difference which can cause huge differneces in handling on TVR!

>> Edited by jam1et on Tuesday 23 March 10:30

[Edt]

I always checked for 22 & 24 psi when cold.. is this wrong ?

Regards, Ed

[RichB]

I run mine at 24 all round, set-cold. Just seems to feel better like that. Rich...

[Guillotine]

ALL tyre pressures are set warm...not hot, unless on track. this is the pressure the tyres will be at whilst driving etc. you could gain 1-2psi on the road easily. also if set cold, ambient temp is far more of a factor. mid winter -2=low, mid summer +26=high.

personal preferences aside.

RichB..i also run my proxies +2...on the track, as the side walls are softer than bridgies.

good info about your own gauge...i ALWAYS use one!

[simpo two]

22 and 24 here. I'd NEVER trust a high street 'Mondeo' place to know anything about a TVR. But there are plenty of other things to check - if you're worried, get the geometry checked by someone competent.

[wixer]

Guillotine said:

ALL tyre pressures are set warm...not hot, !

This is unusual, most, if not all car manufacturers state in their handbooks that tyre pressures should be checked when cold. My Griff handbook doesn't state hot or cold but I always check and adjust them cold.

[simpo two]

There was quite a technical thread lastyear about the merits of measuring tyre pressures when hot/cold etc. Someone with greater knowledge of physics than me worked out that the difference is too small to bother about, about 0.6% if my memory serves me right.

[GarryM]

Err, I don't think so. The difference between hot and cold is significant, factor in water/vapour and it's even more so.

[shpub]

The problem is how hot is hot? Cold measurements tend to be more consistent, easier to do (more lkikley to get checked) and for most people more than adequate.

[simpo two]

GarryM said:

Err, I don't think so. The difference between hot and cold is significant, factor in water/vapour and it's even more so.

I was just remembering this:: www.pistonheads.com/gassing/topic.asp?p=1&f=9&t=67049&h=0&hw=tyre+pressure

Tuesday 2nd December 2003
------------------------------------------------------Quick thought on the cold/hot discussion. As has been pointed out, from the ideal gas law (PV=nRT) pressure in a tyre will increase more or less directly in proportion to the increase in temperature (ignoring the tyre's volume increasing a little). The T in that forumla is an absolute temperature, i.e. T=0 is absolute zero. So if you want to calculate the % increase in pressure you just divide the Hot temp by the Cold Temp and subtract 1. Easiest to use the Kelvin temperature scale, so just add 273 to the temp measured in centrigrade. As an example lets say the air in the tyre starts at 10 degrees centigrade and rises to 25 degrees during the drive. pressure increase would be:-

pressure Inc = (273 + 25) / (273 + 10) - 1 = 0.053 or 5.3%

So if the tyre started at 24PSI it would have risen to 25.3PSI

I would guess from normal road driving that those numbers are a reasonable estimate, and you can thus see that the pressure variations are small enough not to worry about.

Formula 1 tyres are designed to operate at around 100C. If you managed to get your road tyre up to that temperature, the forumla above implies your pressure would go up 8 or 9 PSI.'

[griffter]

I'd have thought as long as the pressures were in the right ball park, of greater importance is getting the pressure equal on axle pairs / all four tyres. This must be easier done with tyres at equal temps - which can't be guranteed if the tyres are (not necessarily equally) warm.

[DustyC]

nsparey said:

Popped into the local ATS today to get the exhaust checked out and one of the front tyres seemed to wearing on the edges. Exhaust was ok, but they checked the tyre pressures and found the front one only had 18 lbs in it, he checked the others and said they were all well down on what they should be, he reckoned 32lb in the front and 28lb in the back. I then set off to our other branch which happens to be 26 miles of wonderful winding roads over the Preseli Mountains. I instantly noticed that the car was much sharper and I could put more power down coming out of corners and it felt much more stable at speed. I notice this evening that the service sticker on the windscreen says 22lb front and 24lb rear, so now I am confused, what should they be? As it’s an SE, I have Toyo Proxies 225/16/50 front and 245/16/45 on the rears. >>> Edited by nsparey on Monday 22 March 23:27

Just had new S03s fitted and the garage inflated them to 35PSI all round. Felt like driving on ice!